Dynamic Pressure Control in Double Loop Reactor

ABSTRACT

The present invention discloses a slurry loop reactor comprising at least two loop reactors connected in series and wherein the line connecting the two loops is subject to a dynamic pressure difference.

This invention is related to the field of olefin polymerisation indouble loop reactors.

High density polyethylene (HDPE) was first produced by additionpolymerisation carried out in a liquid that was a solvent for theresulting polymer. That method was rapidly replaced by polymerisationunder slurry conditions according to Ziegler or Phillips. Morespecifically slurry polymerisation was carried out continuously in apipe loop reactor. A polymerisation effluent is formed which is a slurryof particulate polymer solids suspended in a liquid medium, ordinarilythe reaction diluent and unreacted monomer (see for Example U.S. Pat.No. 2,285,721). It is desirable to separate the polymer and the liquidmedium comprising an inert diluent and unreacted monomers withoutexposing the liquid medium to contamination so that said liquid mediumcan be recycled to the polymerisation zone with minimal or nopurification. As described in U.S. Pat. No. 3,152,872, a slurry ofpolymer and the liquid medium is collected in one or more settling legsof the slurry loop reactor from which the slurry is periodicallydischarged to a flash chamber thus operating in a batch-wise manner. Themixture is flashed in order to remove the liquid medium from thepolymer. It is afterwards necessary to recompress the vaporisedpolymerisation diluent to condense it to a liquid form prior torecycling it as liquid diluent to the polymerisation zone afterpurification if necessary.

Settling legs are typically required to improve the polymerconcentration in the slurry extracted from the reactor; they presenthowever several problems as they impose a batch technique onto acontinuous process.

EP-A-0,891,990 and U.S. Pat. No. 6,204,344 disclose two methods fordecreasing the discontinuous behaviour of the reactor and thereby forincreasing the solids concentration. One method consists in replacingthe discontinuous operation of the settling legs by a continuousretrieval of enriched slurry. Another method consists in using a moreaggressive circulation pump.

More recently, EP-A-1410843 has disclosed a slurry loop reactorcomprising on one of the loops a by-pass line connecting two points ofthe same loop by an alternate route having a different transit time thanthat of the main route for improving the homogeneity of the circulatingslurry.

The double loop systems are quite desirable as they offer thepossibility to prepare highly tailored polyolefins by providingdifferent polymerising conditions in each reactor. It is however oftendifficult to find suitable space to build these double loop reactors asin the current configuration they need to be close to one another inorder to insure adequate transfer of growing polymer from one loop tothe other. The average velocity of the material circulating in thetransfer line is of less than 1 m/s: these lines must therefore be veryshort in order to avoid sedimentation and clogging due to thepolymerisation of residual monomers. There is thus a need to providemeans either to connect two existing reactors that may be distant fromone another or to build two new reactors that do not need to be close toone another if available space so requires.

It is an aim of the present invention to provide control means forconnecting two or more loop reactors.

It is also an aim of the present invention to decrease the residencetime of the material in the line connecting the reactors.

It is yet another aim of the present invention to improve thehomogeneity of the flow in the loop reactors.

It is a further aim of the present invention to increase theconcentration of olefin in the first reactor.

It is yet another aim of the present invention to increase the solidscontent.

Accordingly, the present invention discloses a slurry loop reactorcomprising at least two loop reactors connected in series and whereinthe line connecting the two loops is subject to a dynamic pressuredifference.

It is difficult to maintain a constant pressure difference between thetwo loop reactors, since the control applies to a mixture of batchdischarge and continuous operations. The present invention thusdiscloses a system wherein the pressure in the second reactor iscontrolled in real time by the pressure variations in the first reactorin order to maintain a predetermined pressure difference.

The present invention provides a method for the slurry polymerisation ofolefins that comprises the steps of:

-   -   providing at least two loop reactors connected in series;    -   providing a line connecting two loop reactors wherein the line        connecting two loops is subject to a dynamic pressure        difference;        characterised in that the pressure in the second reactor is        controlled in real time by the pressure variations in the first        reactor in order to maintain a predetermined pressure        difference.

Typical pressure differences are of at most 5 bars, preferably from 0.5to 2 bars and more preferably from 1.5 to 2 bars. It must be noted thatat the end of each dump the pressure difference between the two loopsmay be greater than or equal to the differential set-point value.

In a first embodiment according to the present invention, the two loopreactors are linked by a conventional line connecting the settling legsof the first reactor to the second reactor.

In another, preferred, embodiment according to the present invention,the two loop reactors are linked a by-pass line (11), as represented inFIG. 1, for connecting two points of the same loop reactor (12) and (13)by an alternate route having a different transit time than that of themain route, said by-pass line (11) also collecting the growing polymerexiting the first loop reactor (1) at exit points (14) and sending saidgrowing polymer to an entry point (13) in the second reactor (2).

LIST OF FIGURES

FIG. 1 represents a double loop reactor configuration wherein the tworeactors are connected by a by-pass line.

FIG. 2 represents typical pressure profiles, expressed in bars, in thefirst and second reactors as a function of time expressed in h:min:s.

FIG. 3 represents a double loop configuration including the low pressurereadings indicated as LPn and the system of valves that can be activatedin order to control operations in the reactor.

In all embodiments, pressure is typically controlled by interactionbetween a set-point value and dumping of the legs. Each time theset-point value is reached one leg is dumped and consequently, pressuredrops to a value that is lower than the set-point value: this isessential to maintain control of the pressure. If the pressure drop isnot sufficient, there exists a scenario for recovering control. Thistype of control is necessary for linking leg dumping that is abatch-wise process, to polymerisation in a loop reactor that is acontinuous process.

In prior art, the conventional way of operating the double loop reactorwas to work with a static set-point value and with a static differentialpressure.

The present invention links the set-point value of the second reactordirectly to the process value of the first reactor. It uses a dynamiccontrol system that is able to link the batch-wise dumping process inboth reactors to the continuous polymerisation process.

This invention thus allows maintaining the desired differential pressureat all times.

As a consequence, the dumping dynamic of the second reactor needs to bemore constrained than that of the first reactor in order to cope withall the upsets of both the first and second reactors. Typical pressureprofiles in the first and second reactors are represented in FIG. 2.

When one leg of the first reactor dumps, the pressure drops in the firstreactor and the pressure in the second reactor directly increases whilemaintaining the differential pressure. When the second reactor reachesthe set-point value, that is the actual process value of the firstreactor, thus now at a lower pressure than the initial pressure, minusthe differential pressure, a leg of the second reactor is fired.

The cycle then re-starts and the pressure in the first reactor increasesagain.

The growing polymer exiting the first reactor can be collected either bycontinuous discharge or by settling legs technology. Preferably,settling legs are used.

Throughout the present description the loops forming the slurry loopreactor are connected in series and each loop can be folded.

Optionally, the lines may be jacketed.

When a by-pass line is used, the velocity of the material circulating inthe line connecting the loop reactors, must be sufficient to avoidsedimentation and possibly clogging: it must be of at least 3 m/s.

The present invention may be used with all types of catalyst systems. Itcan be used for the homo- or co-polymerisation of olefins, preferably ofethylene and propylene. It has proven particularly useful for preparingbimodal polymers with metallocene catalyst systems

EXAMPLE Differential Pressure Control.

In normal operations, the pressure in second reactor A was controlled incascade by the pressure of first reactor B through a differentialpressure measurement. This control had a fixed, manually adjustableset-point value that could be varied between 0 and 5 bars.

It was also possible to switch the cascade control on/off manually andstart control separately and independently from the bimodal regulationpackage, in order to allow start-up.

The general set-up is represented in FIG. 3.

In drift cases, several possibilities were considered and studied.

1. The Pressure in Second Reactor A was too High.

Setting the “Minimum Waiting Time Between Dumps” at 1 second shouldprevent the increase of pressure in the second reactor. If however saidpressure had increased and if the additional criteria describedhereafter was not fulfilled then the reactors were killed. Theseadditional criteria were related to the differential pressure.Typically, the differential pressure (DP) set-point value was adjustedbetween 0.5 and 5 bars. If during operation of the reactors, thedifferential pressure dropped below half the set-point value for 30consecutive seconds while the differential pressure control wasactivated, then both reactors were killed. This was handled by the firstdifferential pressure interlock in the cascade.

2. The Pressure in Second Reactor A was too Low.

In this drift case no action was required on differential pressureinterlock: it was covered by the low pressure indicator LP6 representedon FIG. 3. If LP6 was lower than 35 barg, then valves 12, 13, 14 and 15were automatically closed and if it dropped below 30 barg, then valves10 and 11 were automatically locked.

3. The Pressure in First Reactor B was too Low.

This situation occurred for example when a product take-off (PTO) valveremained blocked in open position. The pressure in first reactor Bdropped and so did the pressure in second reactor A. It occurred thatthe pressure difference remained too high to activate the firstdifferential pressure interlock. The Borsig valve above the blocked PTOwas closed if the blockage resulted from a feedback error and ifsimultaneously, the pressure in second reactor A was smaller than 37barg for more than 5 s. This was handled by differential pressureinterlocks 2, 3 and 4 in the cascade.

In another example according to the present invention, low pressure infirst reactor B resulted from low temperature in that reactor. Largeamount of hydrogen in the reactor slowed down the reaction therebyreducing the temperature. If the differential pressure control wasunable to compensate for such pressure drop in first reactor B, thereactor was killed.

4. The Pressure in First Reactor B was too High.

This situation was not critical and it was not necessary to implementany specific action.

1-7. (canceled)
 8. A method for controlling pressure in a double loopreactor comprising: operating a first loop reactor of the double loopreactor at a first initial pressure; operating a second loop reactor ofthe double loop reactor at a second initial pressure, wherein the firstloop reactor and the second loop reactor are connected in series via aline adapted for transferring polymer from the first loop reactor to thesecond loop reactor, and wherein the line is subject to a dynamicpressure difference; linking a set point pressure value of the secondloop reactor to a process pressure value of the first loop reactor; anddischarging polymer from the second loop reactor each time a pressure inthe second loop reactor equals the set point pressure value, wherein theset point pressure value is equal to the process pressure value minusthe dynamic pressure difference.
 9. The method of claim 8, wherein whenpolymer is discharged from the first loop reactor the process pressurevalue of the first loop reactor drops below the first initial pressure,and wherein pressure in the second loop reactor directly increases whilemaintaining the dynamic pressure difference.
 10. The method of claim 9,wherein the polymer is discharged from the first loop reactor and thesecond loop reactor by dumping legs of the first loop reactor and thesecond loop reactor.
 11. The method of claim 8, wherein the processpressure value of the first loop reactor increases after the polymer isdischarged from the second loop reactor.
 12. The method of claim 8,wherein the set point pressure value is between 0 and 5 bars.
 13. Themethod of claim 8, wherein the set point pressure value is between 0.5and 5 bars.
 14. The method of claim 8, wherein pressure in the secondloop reactor is controlled in real time by pressure variations in thefirst loop reactor.
 15. The method of claim 14, wherein pressure in thesecond loop reactor is controlled in cascade by pressure in the firstloop reactor through a differential pressure measurement.
 16. The methodof claim 8, wherein a predetermined pressure difference is maintainedbetween the first loop reactor and the second loop reactor.
 17. Themethod of claim 16, wherein the predetermined pressure difference is notmore than 5 bars.
 18. The method of claim 16, wherein the predeterminedpressure difference is from 0.5 bars to 2 bars.
 19. The method of claim16, wherein the predetermined pressure difference is from 1.5 bars to 2bars.
 20. The method of claim 8, further comprising setting a minimumtime between discharges of polymer from the second loop reactor.
 21. Themethod of claim 8, further comprising: introducing an olefin monomerinto the first loop reactor; contacting the olefin monomer with a firstcatalyst system within the first loop reactor to form a firstpolyolefin; withdrawing the first polyolefin from the first loopreactor; transferring the first polyolefin from the first loop reactorto the second loop reactor via the line; contacting the first polyolefinwith a second catalyst system within the second loop reactor to form asecond polyolefin; and withdrawing the second polyolefin from the secondloop reactor.
 22. The method of claim 21, wherein the second polyolefinis a bimodal polymer, and wherein the first catalyst system and thesecond catalyst system are both metallocene catalyst systems.
 23. Themethod of claim 8, further comprising collecting growing polymer exitingthe first loop reactor by continuous discharge or settling legs.
 24. Themethod of claim 8, wherein the first loop reactor and the second loopreactor are linked by a by-pass line and an alternate route, and whereinthe by-pass line collects growing polymer exiting the first loop reactorat exit points and sends the growing polymer to an entry point in thesecond loop reactor.
 25. The method of claim 24, wherein a velocity inthe by-pass line is larger than 3 m/s.
 26. The method of claim 8,wherein the first loop reactor operates in batch operation.
 27. Themethod of claim 26, wherein the second loop reactor operates incontinuous operation.